Friction

When one body makes an attempt to slide over another body, an opposing force called the force of friction arises as a reaction to the applied force, and acts in the opposite direction. Static friction is the type of friction that exists when two objects touch each other at rest. It has a limiting value called limiting friction, which is equal to the least force required to move the body from rest. When the external force F is increased, a stage comes when the body is just at the verge of moving. At this stage, the force of friction is at the maximum, and is called limiting friction. When the applied force, F, is increased further (beyond the limiting frictional force), the body begins to move, then the force opposing the motion is called the kinetic or sliding friction. The kinetic friction is less than the limiting friction.

This video explains how to (a) study the relationship between force of limiting friction and normal reaction and (b) find the co-efficient of friction between a block and a horizontal surface.